1. Field of the Invention
This invention relates generally to audio equalizers, and more particularly to a graphametric equalizer having characteristics of both graphic equalizers and parametric equalizers contained within a single device.
2. Description of the Prior Art
Audio equalizers are well known in the art. Two well known types of audio equalizers include graphic equalizers and parametric equalizers. Historically, graphic equalizers and parametric equalizers have been considered separate entities; a system designer chooses the type of equalizer that best suits the system needs. Graphic equalizers have a fixed number of filters having fixed center frequencies and bandwidths and adjustable gains. The user is able to adjust these gains for the preferred listening experience. Parametric equalizers, on the other hand, typically have a fixed number of filters wherein each filter has a fixed gain, bandwidth and center frequency. Parametric equalizers are typically used to resolve a specific problem such as undesirable variations in frequency characteristics associated with certain loudspeakers. Parametric equalizers, for example, allow implementation of the filters solely necessary to eliminate or correct such problems.
Application of allpass filter structures to accommodate equalization processes is also well known in the art. FIG. 1, for example, depicts a well known filter structure that can be used to implement second-order equalization filters where A(z) is a second-order allpass filter and k is the peak gain of the filter. The second-order allpass filter A(z) depicted in FIG. 1 further has the form illustrated in FIG. 2. Useful properties of the filter structures shown in FIGS. 1 and 2, such as the relationships that exist between particular parameters and coefficients, are also well known. Boost filters, for example, are known to exhibit the relationships between β and the bandwidth Ω shown in equation 1 and between α and the center frequency ω0 shown in equation 2 below when using such filter structures. Further, cut filters are known to exhibit the relationships between β, the bandwidth Ω and the gain k shown in equation 3 below when using such filter structures.β=[tan(Ω/2)−1]/[tan(Ω/2)+1]  (1)α=−cos(ω0)  (2)β=[tan(Ω/2)−k]/[tan(Ω/2)+k]  (3)
The filter structures shown in FIGS. 1 and 2 have been used by some skilled the art to formulate parametric equalizers and graphic equalizers. Combining features of both parametric and graphic equalizers into a single system to efficiently and reliably accommodate parametric equalization, graphic equalization and other filtering functions that have historically been unavailable, difficult to accomplish, or otherwise of poor quality is a desirable concept. McGrath, D.S., A New Approach to Digital Audio Equalization, 97th AES, November, 1994 discloses however, combining a graphical user interface to a long finite impulse response (FIR) filter equalizer to give some of the benefits of a parametric equalizer with the controllability of a graphic equalizer. McGrath also states that his approach can be accomplished via infinite impulse response (IIR) filters, but gives no details as to how such a structure could be formulated. Further, McGrath does not address issues associated with near real time changes in equalization nor a complete combination of graphic and parametric equalization to accommodate applications that can only be accomplished with such structures.
In view of the foregoing, it would be desirable to have a true combination of graphic and parametric (graphametric) equalizers in a single system without redundant resources to accommodate near real time adjustable parametric equalization among other things.